Surgeons implant a wide variety of metallic, ceramic, and polymeric materials into patients. Surgeons use metallic implants primarily for orthopedic purposes, but additional applications include wound closure (internal and external), reconstructive surgery, cosmetic surgery, wire leads, heart valve parts, aneurysm clips, and dental uses. Because metals have favorable mechanical properties, including elasticity, deformability, and stability, metallic implants are generally less bulky than their non-metallic counterparts—an important precondition for application to minimally invasive surgery. Metallic implants must withstand and function within the body environment at least for a certain period of time. Therefore, the rate and type of structural degradation, via corrosion and other processes while in vivo, is an important consideration in the design of surgical implants. In addition, corrosion of metallic implants is an important consideration for biocompatibility, due to the release of metal ions into the body environment.
Some of the metals currently used for surgical implants include stainless steel (AISI type 316L), cobalt-chromium-molybdenum-carbon, cobalt-chromium-tungsten-nickel, cobalt-nickel-chromium-molybdenum, titanium, Ti-6Al-4V, Ti-3Al-2.5V, and tantalum. These metals transition from an active to a passive state by developing a protective surface oxide film when used as implants and are highly corrosion resistant in saline environments such as in the body.
The body recognizes surgical implants as foreign objects, potentially leading to local and possibly systemic reactions. Permanent metallic implants are particularly undesirable for young patients because retention for decades is unavoidable. Some metallic implants including, for example, surgical staples, clips, and vascular stents, may be constructed of metals that corrode quickly in the body. The corrosion by-products are harmlessly absorbed by the body or passed through the digestive system. For example, a surgical staple made from commercially pure iron may corrode in animal soft tissue within a few weeks, but the staple would have sufficient structural integrity for a long enough period of time, usually several days, to allow healing of the tissues involved. The surgical staple may also be made of other absorbable metals, including carbon steel. The absorption of small amounts of corrosion by-products (for iron or carbon steel, the primary by-product is iron oxide or rust,) is not known to have any significant, deleterious effect on the body. The ferromagnetic property of iron and carbon steel is a factor relative to their compatibility with MRI (magnetic resonance imaging), although the very small mass of some implants, such as surgical staples, and the very short time they are present in the body before corroding and being absorbed, allows the beneficial use of such materials. Other benefits of absorbable staples include reducing scatter on X-ray images, minimizing future adhesions, and avoiding staple lines in future surgical procedures.
Corrosion is primarily the result of an electrochemical reaction of a metal with its environment and occurs because the metal oxide or corrosion product is more stable thermodynamically than the metal. Electrochemical deterioration of the metal occurs as positive metal ions are released from the reaction site (anode) and electrons are made available to flow to a protected site (cathode). The electrochemical reaction cell consists of two conducting and electrically connected electrodes in an electrolytic solution. The two electrodes can be dissimilar metals, or they can result from different surface areas of the same metal, defects, impurities, precipitate phases, concentration differences of gas, solution or metal ions, or other variables. The rate at which the corrosion reaction proceeds is primarily related to environmental composition and effects, such as motion or load. The physiological environment in the human body contains chloride ions (Cl−) and is controlled at a pH level of 7.4 and a temperature of 37 degrees C. (98.6 degrees F.). Following surgery, the pH can increase to 7.8, decrease to 5.5, and then return to 7.4 within a few weeks. These variations are caused by infection, hematoma, and physiological solutions administered during and after surgery.
Corrosion resistance of a metal is specific to a number of factors, including composition, changes in metallurgical heat treatment, microstructural phases present, and surface finish. The rate of corrosion of a metal can be slowed or halted by applying a coating, such as a moisture barrier, that shields the metal from the corrosive environment. Conversely, creating an even harsher corrosive environment can accelerate the corrosion rate of a metal. In addition, it is possible to cause the corrosion process to be focused on a localized area of the metal. By using these principles and biasing the corrosion process to take place at a desired rate and/or at a desired location of the metal, it is possible to design a metallic, surgical implant that corrodes within the body in a beneficial manner.
Each of the many surgical implants that may be made from an absorbable metal has a shape that is designed specifically for its deployment into tissue and its initial, primary function, such as holding tissue layers together during wound healing. As the implant corrodes, the ability of the implant to perform its primary function degrades. Biasing the corrosion rate and location on the implant allows the implant to fragment in a desirable way during the early stages of the corrosion process. For example, physical attributes of the implant important for deployment into tissue are not necessarily desirable thereafter while implanted in the body. The sharp tips of a surgical staple are necessary for penetration into tissue during deployment, but can cause prolonged pain or irritation to the patient thereafter. Procedures with such post-surgical complaints by patients include inguinal hernia repair and hysterectomy (in which a male sexual partner experiences the discomfort.) Also, in some situations, it would be advantageous for the implant to corrode in a specific manner, so that the ability of the implant to perform its primary function even improves. For example, surgical staplers commonly referred to in the art as circular staplers are used to perform an end-to-end or end-to-side anastomosis of hollow organs such as the large or small intestines. The surgeon uses the circular stapler to deploy a plurality of tiny, surgical staples evenly spaced apart in a pair of concentric circular staplelines (or more simply, “staple circles”) around a lumen, in order to connect the two organs together in fluid communication. Each staple is formed into a “B-shape” to clinch tissue layers together. A ring of relatively inelastic scar tissue forms over these staple circles. By using surgical staples that initially corrode and fragment from “B-shapes” into “two half B-shapes”, the primary tissue holding function of the staples is not compromised, yet the staple circles are more flexible and easily dilated.
What is needed, therefore, is a surgical implant made of a metal that corrodes and becomes absorbed in the body without compromising the primary function of the implant, and the rate of corrosion on at least a portion of the implant is alterable in order to provide benefit to the patient.